In lithography techniques, a mask is implemented in patterning the layers of a device. During the lithography process, each layer needs to be patterned accurately so that there are no discrepancies in the overlaying of the layers. As such, inaccuracies from the lithography techniques should be minimized.
As technology nodes shrink, extreme ultraviolet (EUV) lithography masks are becoming more prevalent in the lithography process. An EUV photomask is a patterned reflective mask, compared to an optically transparent mask. That is, EUV masks are fundamentally different from traditional optical masks, as they are reflective masks that require a low surface roughness on the order of a few atoms. These types of masks also have stringent flatness and curvature requirements.
As with all masks, EUV masks are fabricated from blanks. However, fabricating EUV masks is particularly difficult, noting that they are reflective and used for small technology nodes. Issues resulting from improper or inaccurate mask fabrication can result in image placement errors, which can contribute to overlay errors. These image placement errors can be caused by discrepancies from the flatness of the blanks. Accordingly, flatness specifications for the mask blanks which cause overlay issues are of critical importance to address.